1. Field of fhe Invention
The present invention relates to an organic electro-luminescence display device, and more particularly, to a thin film transistor and an electro-luminescence display device using the same that are adaptive for improving a credibility of an organic light-emitting cell drive.
2. Discussion of fhe Related Art
Recently, flat display panels with reduced weight and size have been developed to eliminate disadvantages of a cathode ray tube display device. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”) device, a field emission display (hereinafter, referred to as “FED”) device, a plasma display panel (hereinafter, referred to as “PDP”) device, and an electro-luminescence (hereinafter, referred to as “EL”) display device. In addition, there are active studies for making a flat panel display device with high display quality and a large-dimension screen.
In general, a PDP has been highlighted among flat panel display devices as advantageous to have light weight, a small size and a large dimension screen because its structure and manufacturing process are simple. However, a PDP has a low light-emission efficiency and requires large power consumption. Likewise, an active matrix LCD device employing a thin film transistor (hereinafter, referred to as “TFT”) as a switching device has experienced drawbacks in that it is difficult to make a large dimension screen because a semiconductor process is used. In addition, an active matrix LCD device requires a large power consumption due to a backlight unit and has a large light loss and a narrow viewing angle due to optical devices, such as a polarizing filter, a prism sheet and a diffuser.
Meanwhile, an EL display device is largely classified into an inorganic EL display device and an organic light-emitting diode display device depending upon a material of a light-emitting layer. An EL display device also is advantageous in that it is self-luminous. When compared with the above-mentioned display devices, the EL device generally has a faster response speed, a higher light-emission efficiency, greater brightness and a wider viewing angle.
The inorganic EL display device has a larger power consumption than the organic EL display device, and cannot obtain a higher brightness than the organic EL display device and cannot emit various colors such as red (R), green (G) and blue (B) colors. On the other hand, the organic EL display device is driven with a low direct current voltage of tens of volts, and has a fast response speed. Also, the organic EL display device can obtain a high brightness, and can emit various colors of red (R), green (G) and blue (B). Thus, the organic EL display device is suitable for a post-generation flat panel display device.
In general, a system of driving of an organic EL display device can be classified into a passive matrix type and an active matrix type. The passive matrix organic EL display device has a simple structure and a simple fabricating method, but has large power consumption. In addition, it is difficult to make a large dimension passive matrix organic EL display device. Further, the passive matrix organic EL display device has a drawback in that as the number of wirings increases, an aperture ratio becomes lower. On the other hand, an active matrix organic EL display device has an advantage in that it is capable of providing high light-emission efficiency and a high picture quality.
FIG. 1 is a circuit diagram illustrating a pixel of an active matrix organic electro-luminescence display device according to the related art. In FIG. 1, an active matrix organic EL display device has a structure where pixels P are arranged in a matrix type and at an area defined by an intersection between a gate line GL and a data line DL. Each of the pixels P receives a data signal from the data line DL when a gate pulse is applied to the gate line GL, thereby generating a light corresponding to the data signal.
In addition, each of the pixels P includes an EL cell EL having a cathode connected to a ground voltage source GND, and a cell driver 60. The cell driver 60 is connected to the gate line GL, the data line DL and a supply voltage source VDD and is connected to an anode of the EL cell EL to drive the EL cell EL. The cell driver 60 includes a switching TFT T1, a driving TFT T2 and a capacitor C. The switching transistor T1 is turned on when a scanning pulse is applied to the gate line GL, thereby applying the data signal to a first node N1. The data signal applied to the first node N1 is charged into the capacitor C and is supplied to a gate terminal of the driving TFT T2. The driving TFT T2 controls a current amount I fed from the supply voltage source VDD into the EL cell EL in response to the data signal applied to the gate terminal thereof. The data signal is discharged from the capacitor C even though the switching TFT T1 is turned off, so that the driving TFT T2 applies a current I from the supply voltage source VDD until a data signal at the next frame is supplied, thereby keeping a light-emission of the EL cell EL.
The driving TFT of the organic EL display device having a structure that the pixels in FIG. 1 are arranged in a matrix type employs poly-Si or a-Si semiconductor. In general, the driving TFT T2 using a-Si semiconductor in comparison to the driving TFT T2 using poly-Si semiconductor has an advantages of a simple fabricating process, but has drawbacks of a low electron mobility, a low stability and a low credibility. Accordingly, the driving TFT T2 using poly-Si semiconductor is a popular used, but the driving TFT T2 using poly-Si semiconductor has a problem of a kink effect that is not generated in the driving TFT T2 using a-Si semiconductor.
FIGS. 2A and 2B are graphs illustrating driving characteristics of a driving thin film transistor employing amorphous-Si semiconductor and of a driving thin film transistor employing poly-Si semiconductor according to the related art, respectively. First, a relationship among Vds, Ids and Vgs of the driving TFT T2 using a-Si semiconductor is as shown in FIG. 2A. Vds represents a voltage between a source electrode of the driving TFT T2 and a drain electrode of the driving TFT T2, Ids represents a current or an output current of the driving TFT T2 flowing at the source electrode of the driving TFT T2 and the drain electrode of the driving TFT T2, and Vgs represents a voltage between a gate electrode of the driving TFT T2 and the source electrode of the driving TFT T2.
As shown in FIG. 2A, in the driving TFT T2 using a-Si semiconductor, Ids is proportional to Vds in an interval (hereinafter, referred to as “first interval”) having a low Vds (where Vgs is constant), and Ids has a constant value irregardless of Vds in an interval (hereinafter, referred to as “second interval”) that Vds has a bigger value than the first interval. In particular, Ids is proportionally increased in a value of Vgs in the first interval. Accordingly, a driving characteristics of the driving TFT is stable and a credibility is a pretty good in view of a stability of Ids in the driving TFT T2 using a-Si semiconductor.
As shown in FIG. 2B, in the driving TFT T2 using poly-Si semiconductor, Ids is proportional to Vds in the first interval, and in the second interval, Ids increases at a bigger value than the first interval. In addition, if Vds is larger than a designated value, then Ids increase non-uniformly in an area A, which is referred to a kink effect. The kink effect reduces a driving current of the organic EL cell for emitting the organic EL cell, to thereby cause a reduction in a life span of the organic electro-luminescence display device. To reduce the kink effect, a poly-Si TFT having a dual gate structure has been suggested.
FIG. 3 is a planar schematic diagram illustrating a driving thin film transistor employing poly-Si semiconductor having a dual-gate structure according to the related art, and FIG. 4 is a planar schematic diagram illustrating another driving thin film transistor employing poly-Si semiconductor having a dual-gate structure according to the related art. In FIG. 3, a dual gate TFT includes a penetration hole 80 passing through a center of a gate electrode 58. Thus, a poly-Si semiconductor pattern 78 and the gate electrode 58 cross each other at two areas. The penetration hole 80 passing through the center of the gate electrode 58 is adjoined to a drain electrode 74, to thereby implement first and second sub TFTs ST1 and ST2 having different-sized channel C1 and C2 between a source electrode 72 and the drain electrode 74. In particular, a channel length L1 of the first sub TFT ST1 adjacent to the source electrode 72 is formed to be larger than a channel length L2 of the second sub TFT ST2 adjacent to the drain electrode 74. Such a structure has a kink effect reduction phenomenon, but since the channel length is adjusted, the driving TFT becomes oversized. Thus, the structure is inappropriate to a high-resolution display model.
In FIG. 4, an alternative design of a width W of a channel has been suggested in order to solve a problem by the above-mentioned channel length. In particular, a second channel width W2 of the second sub TFT ST2 adjacent to the drain electrode 74 is formed to have a longer length than a first channel width W1 of the first sub TFT ST1 adjacent to the source electrode 72. Such a structure has a kink effect reduction effect and a size reduction effect. However, an electric field is concentrated at a cusp B of the second channel area, to thereby generate a deterioration at the cusp B. Thus, the structures according to the related have a problem in that a stability of the driving TFT is reduced and a life span of the driving TFT is shortened.